The invention relates to an apparatus for electrodynamically separating particles from a gaseous stream comprising high-tension voltage means for charging the particles in the gaseous stream, there being arranged in the gaseous stream at least one collecting element for the charged particles.
The definition of "electrodynamic separation" has been used here to achieve a clear distincion of the invention against conventional electrostatic apparatuses in which separation of uncharged particles is achieved on an electrostatically charged collecting element. Such an electrostatic filter is described, for example, in the U.S. Pat. No. 3,487,610. The filter comprises permanently charged elements which are kept separate from each other by spacing pieces of a dielectric insulation material, e.g. a block of corrugated paper, through the flutes of which the gaseous stream is led. When the spacer block is first applied to the electrically charged surface of an element, the spacer block may be considered as a large electrically uncharged particle which by degrees takes a charge from the permanently charged elements placed on the two opposing sides of the spacing block. Each spacing block incorporated in the filter will get a charge dependent on the element, and a neutral zone is achieved in the middle portion of the spacer block. This signifies that the corrugated kraft paper blocks may be regarded as extensions of the adjacent statically charged elements. Uncharged particles will pass into the flutes of the spacer blocks, and as the blocks are statically charged, the particles will be caught up on the flute walls with the exception of the central and neutral zone in each spacer block, where the particles pass freely through. Particles which have been caught or collected will be quickly charged to the same potential as the associated flutes, attraction forces thereby ceasing, which gives the particles a tendency to loosen and leave the filter.
Even if the electrostatic apparatus described in the U.S. Pat. No. 3,487,610 constitutes an improvement over other electrostatic apparatuses, it cannot, however, be compared with the considerably more effective but more expensive electrodynamic apparatuses.
In the construction of electrodynamic apparatuses, the collecting elements consist of perforated plates, or the like, of metal with good electrical conducting ability. Such collecting elements, which are earthed with regard to high-tension voltage, constantly lead a current to earth, and catching the particles charged in a high-tension field will be extremely effective. Attempts have also been made to use different semi-conductor material for the collecting elements, but so far such collecting elements have not been brought into commercial use. A disadvantage with both types of collecting element is their weight, the cost of fabricating them, and that as a rule a large number of collecting elements with intermediate electrodes must be provided, since the charge on collected, charged particles gradually decays to earth potential and the particle can then loosen from the surface of the collecting elements. A subsequent collecting element then entrains such particles plucked off by the gaseous stream, and these particles will possibly be united with previously entrained particles to form larger and heavier agglomerates which fall to the bottom of the filter apparatus.
It naturally follows that such a filter will be voluminous, expensive and heavy.
An object of the invention is therefore to achieve an electrodynamic filtering apparatus in which the charge in the charged entrained particles very slowly decays to earth potential, and in which particle lumps having a large mass are quickly formed, whereby the number of collecting elements can be kept to a minimum and a corresponding reduction of the number of high tension electrodes is obtained.
It is further an object of the invention to achieve a light and cheap collecting element consisting of a web of network of paper provided with through-flow channels, on the walls of which the charged particles are collected and retained in the form of lumps and particle layers.
Practical tests have shown that the charged particles caught on such earthed elements fabricated from paper will always generally have a potential differing from earth potential, there being attraction forces between the particles and the channel walls all the while the apparatus is supplied with voltage. The reason for this surprising effect would appear to be that paper is an extremely bad electrical conductor, and that each new charged particle which is collected, and which will then lie in contact with a previously entrained particle, will transfer a portion of its charge to said particle and not only to the collecting element.
According to one embodiment the collecting element or elements incorporated in the apparatus have a thickness of some few centimeters, whereat the channels have a length which is the same as the thickness of the element.
Such an element can suitably have a corrugated paper-like structure, i.e. it can consist of alternate flat layers of paper corresponding to the corrugated paper liner, and corrugated layers corresponding to the corrugated paper fluting. It has been found that most kinds of paper have a sufficient electrical conduction ability for effectively earthing the element. Earthing can be suitably implemented by the periphery of the element being provided with a metal foil which is earthed. The web or network of paper shall thus preferably consist exclusively of paper and the glue required to form the structure resembling corrugated paper. To diminish the risk of combustion, the paper can be impregnated with a fire retarding medium such as waterglass.
An increased separating effect can be obtained by an element of this kind being divided up into two or more thinner part elements, separated by narrow gaps. The increased effect appears to depend on the particles having a tendency primarily to deposit themselves on the edges of the walls forming the channels through the element.
A collecting element of the above described type should be so arranged in the apparatus that it can be removed individually and preferably during operation. Since the element consists of paper it can be burnt together with entrained material. Aternatively, the material can be removed from the element which is then re-inserted in the apparatus.